In the manufacture of insulated electric cables, particularly cables for high performance networks such as cable TV and telecommunications networks, a uniform electrical characteristic for the cable is particularly important to avoid distortions to transmitted signals.
A previously proposed method of monitoring the cable capacitance uniform is to monitor the insulated cable as it emerges from an insulating sleeve extruder machine to determine whether or not the cable characteristics remain within predefined tolerances and/or to control the extruder machine in a sense to keep the characteristics of the cable uniform. The monitoring step typically is carried out using a coaxial capacitance gauge immersed in a bath of cooling water.
As shown in FIG. 1, the gauge consists of a cylindrical measuring electrode 2 flanked by a pair of cylindrical guard electrodes 4 and 6, each guard electrode 4 and 6 lies generally spaced from the adjacent end of the measuring electrode 2 and is coaxial therewith. The cable 8, to be monitored, is guided by guides (not shown) to travel along the common axis of the three electrodes 2, 4 and 6.
The outer diameter of the cable 8 is less than the inner diameter of the electrodes and so the gap between the cable 8 and the electrodes 2, 4 and 6 is filled with coolant 10xe2x80x94usually water. The water also invades the spaces between the guard electrodes 4 and 6 and the measuring electrode 2. Because water has some resistivity, there will, in effect, exist a respective resistor between the measuring electrode 2 and each guard electrode and a resistor R3, R4 and R5 between each electrode and a respective section 8A, 8B and 8C of the opposing outer surface of the cable 8. The core 14 of the cable is a conductor which is normally earthed and so there will, in effect, exist a respective capacitor C1, C2 and C3 between the core 14 and each surface section 8A, 8B and 8C.
FIG. 2 shows the equivalent circuit with the guard to the measuring electrode resistors being connected in parallel and represented as Rm. An oscillator 16 has two output terminals 18 and 20. Terminal 18 is connected to earth or the cable core 14 while terminal 20 is connected to the two guard electrodes 4 and 6.
The measuring electrode 2 and the two guard electrodes 4 and 6 are respectively connected to the two inputs of a differential amplifier 20 with a negative feedback through impedance 22. The feedback loop drives the electrode 4 to keep the electrode 2 at the same potential as the two guard electrodes. The value of the resistors R3, R4 and R5 representing the resistance of the water between the electrodes and the cable 8 are low relative to the impedance of their corresponding capacitors C2, C3 and C1 and the input impedance of the amplifier 20 is sufficiently low that the shunt resistance Rm has negligible effect. The resultant output voltage from the amplifier 20 is then directly related to the capacitance of the section of cable located within the measuring electrode.
This system, however, suffers from the disadvantage that it can only measure the capacitance of a discrete length of the cable ie the length which at any time lies within the measuring electrode. Accordingly, the capacitance is the average capacitance for that length. When the extruder operates in a manner in which it produces a cyclical variations in the capacitance of the cable, the capacitive variation detected is attenuated as a function of the length of the measuring electrode.
It is an object of the present invention to provide an improved capacitance monitoring system.
According to the present invention there is provided a capacitance monitoring system comprising a capacitance head having a generally cylindrical measuring electrode of predetermined length encircling the path of an electric cable, velocity means for measuring the speed of the cable along said path relative to the electrode, means for providing an output signal indicative of the capacitance between the cable and the measuring electrode, a fast Fourier transform device for producing a Fourier analysis on variations in the output signal to indicate discrete cyclical faults in the cable and the frequencies of electrical signals transmitted along the cables to which said faults would be relevant, a reference table device connected to the output of the velocity means and having a reference table providing different correction factors for the attenuation to which the different cyclical faults as measured by the measuring means would be subject and multiplying means connected to the fast Fourier transform device and the reference table device to correct for said attenuation.
According to the present invention there is further provided a capacitance monitoring system comprising a capacitance head having a pair of spaced generally cyclindrical measuring electrodes of different length positioned to encircle and to extend longitudinally of and lie a predetermined distance from the path of a travelling electric cable, velocity means for measuring the speed of the cable along the path relative to the electrodes, first means providing an output signal indicative of the capacitance between the cable and a first one of the electrodes, second means providing an output signal indicative of the capacitance between the second one of the electrodes and the cable, a first fast Fourier transform device for producing a first Fourier analysis of variations in the output signal from the first means to indicate discrete cyclical faults in the cable and the transmission frequencies they would effect when the cable was in use, a second fast Fourier device for producing a second fast Fourier analysis of variations in the output signal from the second means to indicate cyclical faults in the cable and the transmission frequencies they would effect when the cable was in use, reference table means for storing a first table relevant to the first electrode and a second table relevant to the second electrode of correction factors to correct for attenuation in the cyclical faults detected due to the particular length of each electrode, multiplier means connected to the reference table means and the output of the fast Fourier transform devices to correct the attenuation that the detected cyclical faults have suffered due to the lengths of the electrodes with the cyclical faults detected by the first electrode being suppressed or a range of frequencies for which the greatest attenuation occurs, and the correction of faults detected by the second electrode being suppressed over frequencies other than said range.